1. Field of the Invention
The present invention relates to an electric discharge machining apparatus, and more particularly to an electric discharge machining apparatus of the type in which an electric power for electric discharge is accumulated in capacitors to perform electric discharge machining.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing the construction of an example of a conventional electric discharge machining apparatus of the charge storage type. In the figure, reference numeral 1 denotes a charging unit for charging a capacitor 3 through a charging resistor 2 for limiting a charging current. Reference numerals 5 and 6 designate a machining electrode and a workpiece to be machined, respectively.
An operation of the apparatus will be described. In an initial state, the capacitor 3 has no electric charges, and a machining gap is formed between the machining electrode 5 and the workpiece 6 which face each other in a free state. First, a current flows from the charging unit 1 into the capacitor 3 through the charging resistor 2, so that the capacitor 3 is charged. As the charging of the capacitor 3 advances, a voltage appearing across the capacitor 3 rises, and a voltage is applied to the machining gap. Then, discharge will be induced with a certain probability. When discharge occurs, the electric energy stored in the capacitor 3 is supplied to the machining gap through the discharging circuit thereby performing electric discharge machining.
The conventional electrical discharge machining apparatus of this type, which operates as described above has problems described below.
At first, since the probability of discharge occurrence depends on the level of a voltage applied to the machining gap, the machining energy level varies for every discharge, as a result of which the size of a discharge trace formed on the machined surface is also varied. Generally, in electric discharge machining, the surface roughness is determined by the size of the largest discharge trace. On the other hand, as the size of a discharge trace is made smaller, the efficiency of a machining is deteriorated in a greater degree. Therefore, in the above-mentioned circumstances where the size of a discharge trace varies, machining is conducted with forming machining traces that are much smaller than a criterion which is set in view of the required surface roughness. Consequently, this results in that the machining speed is lowered.
The charging rate of the capacitor 3 is determined by the time constant which is determined by the resistance of the charging resistor 2 and take capacitance of the capacitor 3. Generally, in order to increase the machining speed, the frequency of discharge is required to be increased, and the capacitor 3 is required to be rapidly charged. If the resistance of the charging resistor 2 is low and the charge time constant is set to be small in order to increase the charging speed, however, a large amount of current flows from the charging unit 1 into the capacitor 3 before the termination of discharge. This causes a current to directly flow from the charging unit 1 into the machining gap, so that sustaining arc discharge is induced, whereby the machined surface is damaged. Therefore, the resistance of the charging resistor 2 cannot be so low, thereby causing a problem that the machining speed cannot be improved.
Moreover, since the discharging circuit constituted by the capacitor 3 and the machining gap has a very low impedance, electric charges once stored in the capacitor 3 are discharged for a very short period, so that the waveform of a discharging current is changed abruptly. Generally, the steeper a discharging current (especially its rising portion) waveform is, the larger the electrode wear becomes. Accordingly, in order to reduce the electrode wear, it is necessary to make the discharging current waveform gentle. For this purpose, a counter measure may be incorporated in which the discharge time constant is set larger by, for example, connecting an inductance element in the discharging circuit. In order to avoid the sustaining arc discharge, however, it is necessary that the charge time constant is set to be sufficiently larger than the discharge time constant. Consequently, when the discharge time constant is set large, the frequency of discharge is lowered. Therefore, there exists a problem in that it is difficult to reduce the electrode wear while maintaining the machining speed at a practical level.
As a solution of the problems accompanying such a conventional electric discharge machining apparatus of the charge storage type, it has been proposed to provide a plurality of charge and discharge circuits each of which has a charge switch and a discharge switch, as disclosed in, for example, Published Unexamined Japanese Patent Application No. sho-50-101997. Hereinafter, for convenience of discussion, such a machining apparatus is referred to as an electric discharge machining apparatus of the capacitor-switching type.
FIG. 2 is a circuit diagram showing the construction of a conventional electric discharge machining apparatus of the capacitor-switching type. In FIG. 2, the circuit components same as or corresponding to those of the conventional electric discharge machining apparatus shown in FIG. 1 bear the same reference numerals, and the description thereof is omitted.
In FIG. 2, reference numerals 1-1 and 1-2 denote a first charging unit and a second charging unit, respectively; 2-1, a first charging resistor; 2-2, a second charging resistor; 3-1, a first capacitor; 3-2, a second capacitor; 4-1, a first discharge switch; 4-2, a second discharge switch; 13, a charge switch control signal generating circuit; 14-1, a first Zener diode; 14-2, a second Zener diode; 15-1, a first charge switch; and 15-2, a second charge switch.
An operation of the conventional apparatus of FIG. 2 will be described.
In an initial state (immediately after the previous discharge), the second capacitor 3-2 stores no electric charges because of the previous discharge. In this case, the second charge switch 15-2 is maintained in an off-state whereas the first charge switch 15-1 is maintained in an on-state, so that the first capacitor 3-1 is charged by the first charging device 1-1. The amount of electric charges stored in the first capacitor 3-1 depends on the period which has elapsed after the first charge switch 15-1 has been turned on. With the increase of the charging level of the first capacitor 3-1, a voltage appearing across the terminals thereof rises. When this voltage exceeds a Zener voltage of the first Zener diode 14-1, the first Zener diode 14-1 is made conductive and the first discharge switch 4-1 is then operated to connect the first capacitor 3-1 to the machining gap. At the same time, the charge switch control signal generating circuit 13 controls the second charge switch 15-2 to be turned on.
After a while, discharge is induced in the machining gap, the electric charges in the first capacitor 3-1 are consumed, and the first Zener diode 14-1 is made nonconductive. At the same time, the charge switch control signal circuit 13 controls the first charge switch 15-1 to be turned off. During this period the second charge switch 15-2 remains on, and therefore the second capacitor 3-2 is charged by the second charging unit 1-2. The amount of the electric charges stored in the second capacitor 3-2 depends on the period which has elapsed after the second charge switch 15-2 was turned on. With the increase of the charging level of the second capacitor 3-2, a voltage appearing across the terminals thereof rises. When this voltage exceeds the Zener voltage, the second Zener diode 14-2 is made conductive and the second discharge switch 4-2 is operated to connect the second capacitor 3-2 to the machining gap. At the same time, the charge switch control signal generating circuit 13 controls the first charge switch 15-1 to be turned on. After a while, second discharge is induced in the machining gap, and the electric charges in the second capacitor 3-2 are consumed, and the second Zener diode 14-2 is made nonconductive. At the same time, the charge switch control signal circuit 13 controls the second charge switch 15-2 to be turned off. The state of the apparatus is returned to the initial state. By repeating the above-mentioned operation, therefore, electrical discharge machining can be repeated.
In the conventional electric discharge machining apparatus of the capacitor-switching typed which operates in the above-mentioned manner, while one capacitor is charged, electric discharge machining is performed by the other capacitor. Therefore, it can be expected that the machining speed is improved to some degree. However, since discharge is induced with a certain probability, an unfavorable case may occur wherein the voltage appearing across the terminals of one capacitor exceeds the Zener voltage before the electric charges in the other capacitor are not yet completely discharged. If discharge is induced at this time, discharge of the electric charges in the other capacitor is followed by that of the electric charges in the one capacitor. As a result, an unexpected large discharge trace is formed on the surface of the workpiece, whereby the machined surface is damaged. Moreover, the electrode wear is large in a similar manner as the conventional electric discharge machining apparatus of the charge storage type.
When the charge switches operate slowly, a charging current from the DC power supplies flows directly into the machining gap during discharge. This causes the machined surface to be damaged. Therefore, it is necessary to use a charge switch which operates at a high speed, thereby causing a problem that the apparatus becomes expensive.
As described above, conventional electric discharge machining apparatus have the problems that the machining speed cannot be improved and that the electrode wear is large. Moreover, there is the problem that an unexpected large discharge trace is formed on the surface of a workpiece, so that the machined surface is damaged. Furthermore, there is the problem that the use of a charge switch which operates at a high speed causes the apparatus to become expensive.